A Method Based on Time-Scale Factor for Correcting the Nonlinear Frequency Sweeping in an OFDR System

A location deviation compensation algorithm based on maximum cross correlation is proposed for optical frequency domain reflectometry. The characteristic change of fiber under test and the auxiliary interferometer induced by strain/temperature causes location deviation in the optical frequency domain reflectometry system, which in turn leads to demodulation errors. Compared to current available compensated methods, this algorithm simultaneously compensates the location deviation caused by the fiber under test and the auxiliary interferometer. The temperature variation experiments of the fiber under test and the auxiliary interferometer were carried out, respectively, and results show that the algorithm minimized the demodulation errors introduced by the location deviation.

This paper describes an integrated, accurate, and inexpensive semiconductor laser -based optical frequency domain reflectometry (OFDR) system design. The system utilizes the fiber under test for both sensing and frequency sweep linearization functions, allowing the system to mitigate and compensate for phase errors without the need for an auxiliary interferometer, as is the case for traditional OFDR systems. Benefiting from the unique and embedded design, this system reaches the minimal OFDR system with only one optical interferometer and its corresponding optic-electric components without sacrificing accuracy. In addition, conventional design requires an external auxiliary interferometer, which may experience different noises from the main measuring interferometer, deteriorating the overall performance. Experimental results demonstrate the enhanced performance of the compact design as compared with the former methods, as well as the reduced complexity and improved cost-effectiveness.

Photosensitive polymers serve as critical adhesives in optical devices, with their thermal strain coefficient determining the performance of temperature-dependent loss in such devices. To accurately measure the true thermal strain coefficient of the adhesive, we proposed a measurement scheme of internal strain distribution in photosensitive polymers by using an optical frequency domain reflectometry (OFDR) system. In our scheme, a self-referencing approach is used to eliminate the temperature-to-strain crosstalk caused by temperature variations in the tested sample, thereby obtaining the true thermal strain coefficient of the tested sample. In our experiment, a 10 cm single-mode fiber (SMF) is embedded within the 2 mm × 2 mm × 100 mm photosensitive polymer, and we use the high spatial resolution of the OFDR system to realize the distributed strain measurements of the SMF. A thermal strain coefficient of 19.108 μϵ/°C is obtained when the temperature ranges from 30 °C to 120 °C. The temperature-to-strain crosstalk is 9.131 μϵ/°C, and the spatial resolution of the OFDR is 1.28 mm. In addition, we validated the stability of the entire experimental setup by conducting long-term measurements under fixed environmental conditions, during which the system fluctuations remained within approximately ± 3 GHz. The experimental results can provide valuable insights for the selection of adhesives and the determination of optimal curing time of the passive optical devices fabrication in the field of optical fiber communication.

In this paper, an algorithm based on homomorphic deconvolution is proposed to give an accurate estimation of nonlinear phase in the beat signal in optical frequency domain reflectometry (OFDR). Nonlinearities in the beat signal are obtained by using an auxiliary interferometer. After converting to cepstrum domain and filtering, the nonlinearity can be separated from the beat signal. Then, the deskew filter is used to eliminate the nonlinearity. In the proposed algorithm, no approximations are used, so the estimation is theoretically unbiased. Certain simulations are performed to verify the versatility and effectiveness of the proposed algorithm. The nonlinearities are accurately estimated and eliminated by the method, which improves the spatial resolution of the OFDR system.

We present a distributed high sensitivity static strain sensing method based on all grating optical fiber in optical frequency domain reflectometry (OFDR).Through the use of all grating optical fiber, the backscattered light can be greatly improved, which increases the signal-to-noise ratio. A higher signal-to-noise ratio can help to obtain more accurate distance domain information, thereby obtaining better segmented optical frequency domain information for cross-correlation. Therefore, in the same cross-correlation method, our system obtains even larger range of FUT information due to the signal with higher signal-to-noise ratio. Using this system, we measured the change of strain at the end of a total 200 m FUT. The minimum measurable strain variation is 5 με and the spatial resolution of the system is 10cm. It should be noted that we use interpolation zero padding method on the basis of cross-correlation to obtain higher strain accuracy. In addition, we verified the stability of the system by fitting the results of different strains. The linearity of the fitting results is extremely high, and the R-square can reach more than 0.9. Compared with the OFDR system using the single mode fiber, the OFDR system using all grating optical fiber can provide a better sensing performance of measuring a minor strain variation under the same sensing spatial resolution. This is of great significance for the improvement of OFDR measurement applications.

As the security of optical fiber lines in data centers has attracted growing attention, it has become increasingly important to accurately characterize the fiber that is used. Optical frequency domain reflectometry (OFDR) has been demonstrated as a means of identifying specific segments of optical fiber; however, OFDR measurements are limited in length due to initial optical frequency (IOF) variations. This Letter describes a detailed analysis of IOF and introduces a method to mitigate it in an OFDR system constructed using a semiconductor laser (SCL). Additionally, an algorithm is described that minimizes the calculating density necessary for OFDR-based optical fiber verification, reducing the calculating time required by an order of magnitude relative to prior techniques. Experiments demonstrate that the described method can be effectively applied to a range of application areas, ranging from centimeter to meter lengths of optical fiber, with an error equal rate (EER) of less than 1%.

In modern society, the amount of data being transferred is growing at an exponential rate, about 2.5 quintillion bytes data created per day. Such data can be realized in the form of include the audio, video, or text. Furthermore, due to the many advantages of the optical fiber, including the low-cost, immunity to electromagnetic interface, light weight, and high carrier frequency, it is taking over the industry. The question then arises as how to diagnose the inner fault and maintain the integrity becomes the crux for the telecommunication system based on optical fiber. There are two main high-precision measurement methodologies to complete the task mentioned above: one is the optical time-domain reflectometry (OTDR), which has been widely applied in technologies such as light detection and ranging (LiDAR) in faraway situations. The other one is optical frequency-domain reflectometry (OFDR), which is also referred to as optical frequency modulated continuous wave (OFMCW) or chirped frequency measurement. Compared with the OTDR, the OFDR measures distance by comparing the phase change of the chirped incident optical signal and reflected signals to capture the distance information. Even though the OFDR has been proven to have many advantages over OTDR in terms of the high spatial resolution at μm level, low power consumption, and eye-friendliness, both the high complexity and cost significantly limit its applications. To overcome the above shortages in the OFDR system and expand this technology in the various applications, the following research was conducted. Firstly, since the optical tuning source utilized in OFDR is too expensive and complex to be implemented in-situ measurements, there are some researches completed in the tuning laser source control methods proposed to reduce the cost and complexity of this technology and fit the various applications. Additionally, to enhance the accuracy of the OFDR measuring, some tuning rate error compensating algorithms were introduced. Benefiting from the improved spatial resolution, the physical unclonable function (PUF) in the optical fiber can be achieved as its unique identification to sustain the data transferring security. Furthermore, even though the initial optical frequency (IOF) in OFDR has been disregarded in the distance measuring, it does influence the PUF reader precision. To fully analyze such effect, some research and IOF mitigating methods were proposed, in which a fast processing method for the PUF reader has been introduced by decreasing the calculating complexity. From the above research, a low-cost, accurate, and effective OFDR system has been

We use a spectrum interpolation technique to improve the distributed strain measurement accuracy in a Rayleigh-scatter-based optical frequency domain reflectometry sensing system. We demonstrate that strain accuracy is not limited by the "uncertainty principle" that exists in the time-frequency analysis. Different interpolation methods are investigated and used to improve the accuracy of peak position of the cross-correlation and, therefore, improve the accuracy of the strain. Interpolation implemented by padding zeros on one side of the windowed data in the spatial domain, before the inverse fast Fourier transform, is found to have the best accuracy. Using this method, the strain accuracy and resolution are both improved without decreasing the spatial resolution. The strain of 3μϵ within the spatial resolution of 1cm at the position of 21.4m is distinguished, and the measurement uncertainty is 3.3μϵ.

A distributed optical fiber grating sensing system with large capacity and high spatial resolution is presented. Since highdensity identical weak grating array was utilized as sensing fiber, the multiplexing number was greatly increased, meanwhile, optical frequency domain reflectometry (OFDR) technology was used to implement high resolution distributed sensing system. In order to eliminate the nonlinear effect of tunable light source, a windowed FFT algorithm based on cubic spline interpolation was applied. The feasibility of the algorithm was experimentally testified, ultimately, the spatial resolution of system can reach mm-level. The influence of the crosstalk signal in the grating array on the OFDR system was analyzed. A method that a long enough delay fiber was added before the first FBG to remove crosstalk signal was proposed. The experiment was verified using an optical fiber with 113 uniform Bragg gratings at an interval of 10cm whose reflectivity are less than 1%. It demonstrates that crosstalk signal and measurement signal can be completely separated in the distance domain after adding a long enough delay fiber. Finally, the temperature experiment of distributed grating sensing system was carried out. The results display that each raster's center wavelength in the fiber link is independent of each other and the center wavelength drift has a good linear relationship with the temperature. The sensitivity of linear fitting is equal to 11.1pm/°C.

A new hardware configuration to correct nonlinear tuning affects in optical frequency domain reflectometry (OFDR) is proposed. Unlike conventional correction methods, the new configuration does not require a separate auxiliary interferometer detection, which may eliminate the need for an extra detection device and an acquisition channel. In the proposed method, an intentional beat signal is introduced at the beginning of the OFDR spectrum, which is treated as an auxiliary interferometer to acquire tunable laser phase information for the nonlinear tuning correction. The proposed method may reduce the overall system cost, reduce the data acquisition time and computational load, and make system configuration simpler by eliminating the need for extra components. In this article, the feasibility of the proposed configuration is demonstrated by correcting nonlinear tuning affects using simulations and experiments in a ∼50 m long optical fiber. Distributed strain and temperature sensing are also performed in a 50 m optical fiber with the proposed new configuration.

The nonlinear phase of beat signal in Optical Frequency domain Reflectometry(OFDR) has become a bottleneck to its utilization. In this paper, a method to compensate nonlinear phase based on Match Fourier Transform algorithm(MFT) is presented. The nonlinear phase of beat signal is represented as a polynomial phase function, then the coefficients of the polynomial phase function are obtained from an auxiliary interferometer through the polynomial regression algorithm. Finally, the nonlinear phase in main OFDR interferometer can be compensated by Match Fourier Transform algorithm. The experimental results show that the method can effectively suppress the side lobe of the reflection peak and achieve a spatial resolution of 0.2m at 4.3km. The experiment results verify the effectiveness of the proposed algorithm.

With a rising trend to use optical fiber in both short-reach and long-haul network applications, it has become necessary to detect faults with high spatial resolution, sensitivity, and dynamic range in industry. Unfortunately, the most widely used diagnostic technique, optical time-domain reflectometry (OTDR), has an inherent trade-off between the above mentioned figures of merit. Consequently, traditional OTDR systems can either be used in short-reach applications with high spatial resolution or long-haul applications with high dynamic range. Both OTDR and optical frequency-domain reflectometry systems have been proposed in the past which can arguably close the gap between the two extremes, but they also have a trade-off with the cost, form-factor, and complexity. Recently, researchers have demonstrated an input/output (I/O) interface integrated OTDR (iOTDR), which uses digital I/O ports to perform OTDR measurements with high spatial and voltage resolution. The iOTDR eliminates the necessity of using high-end analog-to-digital converters and demonstrates its potential to be fully integrated in an optical switch with minimum hardware modification. Additionally, the iOTDR is also reconfigurable and software-defined, making it a power- and resource-efficient solution. Thus, it is especially attractive for short-distance communication links, such as those in a datacenter, where computational resources are limited. This manuscript expands upon the advantages brought upon by the iOTDR to propose and demonstrate a new versatile iOTDR that can achieve high spatial resolution, sensitivity, and dynamic range for both short-reach and long-haul networks thanks to its highly reconfigurable design.

We demonstrate a novel method for obtaining high spatial resolution optical frequency domain reflectometry (OFDR) utilizing recirculating frequency shifter for broadening the optical frequency sweeping. 10-times broadened optical frequency sweeping is achieved. 1.46-cm spatial resolution is obtained with modulation frequency sweeping span of 712 MHz.

We introduce a new optical frequency domain reflectometry (OFDR) system and processing method that utilize negative beat frequencies for the first time. The new approach enables efficient use of the available system bandwidth and facilitates distributed sensing with the maximum allowable update rate for a given fiber length. This is achieved by using a coherent optical-communications-type receiver that detects both the in-phase (I) and quadrature (Q) components of the backscatter field. The I and Q components are digitally combined to produce a complex backscatter signal whose Fourier transform is not necessarily symmetric. Judicious processing of the complex backscatter signal maps the reflection profile of one half of the sensing fiber to positive beat-frequencies and the profile of the other half to negative beat-frequencies. The new approach was tested via comprehensive computer simulations and experiment.

In order to eliminate the influence of laser frequency nonlinearity, the frequency-scanning interferometry (FSI) often uses the beat signal of an auxiliary interferometer as the external clock. The time points at every amplitude peaks and bottoms of the auxiliary beat signal are selected as the sampling time points for the main interferometer signal. To satisfy the Nyquist sampling requirement, the optical path difference (OPD) of the delay fiber in auxiliary interferometer should be at least twice longer than the measurement distance. In this paper, we proposed a method to shorten the length of delay fiber. The Hilbert transform was used to extract the phase of the auxiliary interference signal and calculate the time points corresponding to subdivided phase intervals. Then, the main interference signal was resampled at these moments, and the fast Fourier transform was performed on the resampled signal. The experimental results showed that the target at the distance of about 5 m was measured when the OPD of the auxiliary interferometer was about 4.5 m. The standard deviation of the distance measurement results could reach 4.64 μm.